Bistable positioner comprising a dimensionally bistable ferroelectric crystal

ABSTRACT

An electroded crystal element cut along the crystallographic a or b axis from a single crystal of gadolinium molybdate will change its physical dimensions when its spontaneous polarization is changed by the momentary application of an electric pulse. The crystallographic a and b axes interchange with the change in the direction of spontaneous polarization. The change in length of the crystal element may be used to actuate panel display devices and to provide bistable type relay. The bistable relay described herein provides a function similar to a latching relay in that no input power is required to maintain the relay in either of two stable states.

United States Patent Cummins Aug. 12, 1975 BISTABLE POSITIONER COMPRISING A 2,708,243 5/1955 Brajer H 10/8 3,437,432 4/1969 Borchardt 310/935 x DIMENSION ALLY BISTABLE FERROELECTRIC CRYSTAL Primary Examiner-Mark O. Budd [76] Inventor: Stewart E. Cummins, H810 Stafford New Carlisle Ohio 45344 grlgrlfsezy Agent, or irm Robert K. Duncan. Joseph [22] Filed: June 19, 1972 [21] Appl. No.: 263,896 [57] ABSTRACT Rela'ed A n fi Data An electroded crystal element cut along the crystallo- [63] Continuation of Set No M3 598 May 4 971 graphic a or b axis from a single crystal of gadolinium abandoned which is a confi'nualion of molybdate will change its physical dimensions when its gmmf,v A 3 W69 abandoned spontaneous polarization is changed by the momentary application of an electric pulse. The crystallo- 52 us. c1. 310/8; 310/31, 310/83; graphic a and b axes interchange with the in 310 95; 252 29 the direction of spontaneous polarization. The change 51 Int. Cl "01v 7/00 in length of the Crystal element y be used to actuate [58} Field of Search 252/629, 3lO/8, 8.1, 8.3, Panel display devices and to Provide bistable yp 310 9 95 lay. The bistable relay described herein provides a function similar to a latching relay in that no input 5 References Cited power is required to maintain the relay in either of UNlTED STATES PATENTS able 2,661,432 1211953 Mumper IMO/8.7 X 9 Claims, 10 Drawing Figures C i A 6 (6 TIGOD j ("a 00 CRYJTHL t 4 F0! N gtza'rkoog Dow BISTABLE POSITIONER COMPRISING A DIMENSIONALLY BISTABLE FERROELECTRIC CRYSTAL BACKGROUND OF THE INVENTION The field of the invention is in the electromechanical transducer art and more particularly in bistable mechanical positioning devices.

Prior art mechanical and electromechanical position ers for display panel devices are well known. Piezoelectric and magnetostriction devices are currently widely used. A typical example of such prior art is exemplified by U.S. Pat. No. 3,376,092 issued to patentees D. S. Kushner et al. These prior art devices do not have bistable characteristics. They require a continuous excitation to be maintained in an actuated state.

Latching relays are well known. They have depended upon a mechanical arrangement for maintaining the relay armature in the switched state and in general required considerably more excitation power than conventional relays. The bistable relay described herein provides a function similar to a latching relay in that no input power is required to maintain the relay in either of two stable states.

SUMMARY OF THE INVENTION An electromechanical bistable positioner is provided that requires only switching pulses to establish a mechanical position with no further electrical energy input being required to maintain the device in the new position.

BRIEF DESCRIPTION OF THE DRAWING FIG. I shows the change in the crystallographic axes of a gadolinium molybdate crystal with a change in its direction of spontaneous polarization brought about by an electric pulse changing the polarity of the electric field;

FIG. 2 shows the typical hysteresis characteristic of gadolinium molybdate crystals;

FIG. 3 shows the physical changes in a crystal element occurring with the interchange of the a and b axes;

FIG. 4 shows the angular motion of a square element of a crystal cut at 45 degrees to the a and b axes with changes in spontaneous polarization;

FIG. 5 shows how a crystal rod element is cut from a crystal for optimum length changes with changes in spontaneous polarization;

FIG. 6 is a representative view of an embodiment of the invention for providing bistable positioning through changes in the length of the crystal element with changes in spontaneous polarization;

FIG. 7 is an embodiment ofa read-out display device;

FIG. 8 is a front view of the display panel of the embodiment of FIG. 7;

FIG. 9 is a representative view of an individual positioner assembly unit of the embodiment of FIG. 7; and

FIG. 10 is a representative view of an embodiment of a two-position bistable relay.

DESCRIPTION OF THE PREFERRED EMBODIMENTS When the ferroelectric polarization (along the caxis) of a crystal of the ferroelectric material Gd M00411 is reversed the orthorhombic a and h axes in the crystal interchange. This is shown in FIG. I. The direction of the spontaneous polarization P is reversed by applying an electric pulse to the electrodes contacting the crystal. The electric field thus momentarily formed by the pulse will reverse the spontaneous polarization of the crystal. The two stable states of spontaneous polarization are shown at 21 and 22 on the hysteresis characteristic of the material shown in FIG. 2.

The interchange of the a and b axes in the crystal with a change in the direction of spontaneous polarization results in a physical change in the shape of the crystal as shown in FIG. 3. This change in shape of the crystal is a bistable effect since it is related to the bistable ferroelectric polarization. This ferroelectric polarization is easily reversed, that is changed from point 21 to point 22 on the hysteresis curve of the material, by the momentary application of an applied voltage (of the correct polarity) to the electrodes on the crystal. The application of electrodes to crystal elements is well known and will not be further discussed. It is to be understood that in the common usage of the term crystal what is generally implied is a crystal element cut from a single crystal of the material.

The change in shape of a crystal element of Gd (MoO with a change in the direction of spontaneous polarization is quite large. For an approximately square crystal element cut at 45 to the a and b axes and supported along one edge as shown in FIG. 4, the angle of motion 0 has been found to be approximately 03, with reversals of the spontaneous polarization.

A crystal element having optimum linear motion along its length direction is cut along the crystallographic a or b axis of the crystal with parallel surfaces normal to the c axis. This is shown in FIG. 5 with the element 31 to be cut from the crystal blank 32. The element could equally as well be cut along the other orthorhombic axis (whether it be the a or b axis depends on the particular spontaneous polarity then possessed by the crystal) as shown at 33. Such a crystal element is shown in FIG. 6 positioned in fixed relation at one end 41 with motion AL occurring at the other end 42. The fixed end may be conventionally cemented to a supporting surface and the object to be positioned cemented to the movable end of the element, taking care not to short the electrodes. Typical values of AL have been found to be from 0.2 to 0.3 percent of the element length L. In a particular operating embodiment a crystal element of Gd (MoO.,) having end faces 41 and 42 two millimeters square normal to the a or b axis and a length along that axis of 4.28 millimeters changed length by approximately .013 millimeter with reversals of spontaneous polarization. The voltage across the electrodes to establish the electric field reversing the spontaneous polarization was approximately 1200 volts. This is the magnitude of the plus or minus voltage, with respect to ground, as shown at V in FIG. 6, to effect the aforementioned length change AL of 0.013 millimeter in this particular embodiment. The value of the coercive field for gadolinium molybdate is approximately 5 kV/cm. making necessary a potential V,- of approximately I000 volts to switch the 2 millimeters thick crystal element described. Allowing an additional 200 volts as a factor of safety to insure complete reversal of the crystal polarization gives a practical switching potential of I200 volts. Both V and V, are illustrated in FIG. 2. The spontaneous polarization value is approximately 0.20 microcoulombs per square centimeter.

For maximum change of the physical dimensions of the crystal element the direction of the spontaneous polarization along the c axis is completely reversed. Lower values of electric fields occasioned by using lower values of voltage will reverse the spontaneous polarization over only part of the crystal element leaving a domain wall between the two parts of the crystal element with correspondingly lesser amounts of physical change in the crystal. As in the instance of maximum length change, partial changes in length are stable and the crystal will remain in that particular physical state after removal of the electric field. Generally the device is used as a bistable positioner with homogeneous spontaneous polarization throughout providing maximum length changes.

FIG. 7 is a representative view of an embodiment of an improved indicating display device. The length of the crystal rod bistable positioner elements 51 depend upon the state of their spontaneous polarization. This in turn determines whether the supported surface 57 at the movable end of the positioner contacts the back surface of the display panel 64 or not, which determines the read-out of the device as shown in FIG. 8. The amount of motion of the movable end of the positioner may readily be varied by varying the length of the element. The optics of such display devices and the electrical circuitry for actuating a matrix of the positioners are well known. The novelty herein disclosed resides in the providing of bistable positioner elements that require only a pulse of electricity to change their physical state, while the prior art devices have required continuous electrical excitation to maintain them in at least one of their switched states. FIG. 9 is a more detailed view of an individual positioner as employed in the device of FIGS. 7 and 8. The crystal element 51 has conventional electrodes 52 and 53 on the parallel surfaces of the element normal to the crystallographic c axis with electrical leads and connectors 54 and 55 attached thereto. The movable end 56 of the element 51 is ground flat and coated with a suitable material 57 that can be finished to an optical flat surface. Such materials and techniques are well known in the optical art. The opposite end 58 of the crystal element is supported by cementing to support member 61 of FIG. 7. The support member 61 may be an electrical conductor so it may also serve as a common electrical contact for one side of all the positioner elements with the other electrical connection to each element brought out to separate terminals on terminal board 62. Normal care should be used not to short out the electrodes on the crystal. The electrode surfaces may be stopped short of the ends as shown in FIG. 9 to preclude shorts or flashover. or suitable insulation may be used between the crystal element and the attachments to the ends of the crystal element. Support member 61 of FIG. 7 may be fabricated from conventional insulating material and pairs of leads from each crystal element brought out to separate terminals if the common connection described above is not desired. Bipolar DC source 65 and singlepole double-throw normally-open switches 66 provide for the momentary application of voltages of either polarity to the electrodes of the bistable crystal elements 51 for selectively switching these elements between their two stable dimensional states.

An embodiment of a bistable relay is shown symbolically in FIG. I0. The bistable crystal element 71 has electroded surfaces 72 and 73 normal to the crystallographic c axis. Electrical leads 74 and 75 contacting the electroded surfaces conduct the relay switching pulses from the terminals 76 and 77 to the electrodes. Terminal 76 may be placed at electrical ground potential. then positive and negative pulses at terminal 77 will switch the relay. Such pulses may be supplied as in FIG. 7 by bipolar DC source 65 and a single-pole doublethrow normally-open switch 66. An electrical conducting surface 78 mounted on insulating member 85, a conventional printed circuit board may be used as an assembly, is conventionally cemented with cement 79 to the movable end of the bistable crystal element. The fixed end 82 of the crystal element is attached to insulating support member 84 with cement 83. Conventional relay contacts 80 and 81 make electrical contact with conducting surface 78 when the crystal element is extended, closing the controlled circuit, placing it in the on" state. When a pulse of the opposite polarity is applied to terminal 76 (or the polarity between terminals 76 and 77 reversed) the crystal element 7] will contract along its length opening the contacts 80 and 81 placing the controlled circuit in the off" state. As the crystal element is bistable the relay will remain in either state until pulsed again with a pulse of the opposite polarity. As previously explained the length of the crystal element will determine the travel of the conducting surface. The magnitude of the switching voltage required for maximum change of length (complete reversal of the spontaneous polarization of the crystal) is dependent upon the thickness of the crystal element along the c axis and may be determined as previously set forth.

Other ferroelectric crystal elements of the molybdates of the rare-earths, such as Tb M000 may be used in place of Gd M00 in similar embodiments of the invention.

1 claim:

I. The method of bistably positioning a first element with respect to a second element comprising: attaching a crystal of a molybdate of a rare earth at one point on the crystal to the first element and at another point on the crystal to the second element, said crystal having orthorhombic crystallographic axes conventionally designated a, b, and c and said points of attachment lying on a line parallel to one of the a and b axes; and momentarily subjecting said crystal to an electric field that is parallel to the c axis, of polarity determined by which of the two stable positions is occupied by the first element, and of magnitude exceeding the coercive field of the crystal.

2. The method of claim I in which said crystal is a crystal of Gd (MoO 3. The method of claim 1 in which said crystal is a crystal of Tb (MoO,)

4. Apparatus for bistably positioning a first element relative to a second element comprising: a crystal of a molybdate of a rare earth attached at one point on the crystal to the first element and at another point on the crystal to the second element. said crystal having orthorhombic crystallographic axes conventionally designated a, b, and c and said points of attachment lying on a line parallel to one of the a and b axes; and means for momentarily subjecting said crystal to an electric field in a direction parallel to the c axis, of magnitude ex ceeding the coercive field for the particular crystal, and of either polarity.

first element and a second part constituting said second element which parts cooperate to provide visually distinguishable indications of the two stable positions of the first element relative to the second element.

9. Apparatus as claimed in claim 4 in which said first element is an electrically conductive means for closing an electric circuit in one of the two stable positions of the first element relative to the second element and for opening the circuit in the other stable position.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION Page 1 of 2 PATENT NO. 3,899,697

DATED August 12, 1975 Stewart E Cummins It is certified that error appears in the above-identified patent and that said Letters Patent INVENTOR(S) I Pal-Q 1 d m n m 80 m h ..s m 7 s s 5 a m 1+ w d C !a m m w s Y ed e J m m6 w ih 8 FS Illll I In T 7 UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION DATED August 12, 1975 INVENTOR(S) Stewart E Cummins it is certified that error appears in the aboveidentified patent and that said Letters Patent are hereby corrected as shown below: i

&1

Signed and Scaled this fifth Day Of July 1977 [SEAL] A (test:

RUTH C. MASON C. MARSHALL DANN g ffi Commissioner of Patents and Trademarks 

1. The method of bistably positioning a first element with respect to a second element comprising: attaching a crystal of a molybdate of a rare earth at one point on the crystal to the first element and at another point on the crystal to the second element, said crystal having orthorhombic crystallographic axes conventionally designated a, b, and c and said points of attachment lying on a line parallel to one of the a and b axes; and momentarily subjecting said crystal to an electric field that is parallel to the c axis, of polarity determined by which of the two stable positions is occupied by the first element, and of magnitude exceeding the coercive field of the crystal.
 2. The method of claim 1 in which said crystal is a crystal of Gd2(MoO4)3.
 3. The method of claim 1 in which said crystal is a crystal of Tb2(MoO4)3.
 4. Apparatus for bistably positioning a first element relative to a second element comprising: a crystal of a molybdate of a rare earth attached at one point on the crystal to the first element and at another point on the crystal to the second element, said crystal having orthorhombic crystallographic axes conventionally designated a, b, and c and said points of attachment lying on a line parallel to one of the a and b axes; and means for momentarily subjecting said crystal to an electric field in a direction parallel to the c axis, of magnitude exceeding the coercive field for the particular crystal, and of either polarity.
 5. Apparatus as claimed in claim 4 in which said crystal is a crystal of Gd2(MoO4)3.
 6. Apparatus as claimed in claim 4 in which said crystal is a crystal of Tb2(MoO4)3.
 7. Apparatus as claimed in claim 4 in which said means for subjecting the crystal to an electric field provides selectivity as to the polarity of the field and also provides selectivity as to when the subjection occurs.
 8. Apparatus as claimed in claim 4 in combination with an indicator having a first part constituting said first element and a second part constituting said second element which parts cooperate to provide visually distinguishable indications of the two stable positions of the first element relative to the second element.
 9. Apparatus as claimed in claim 4 in which said first element is an electrically conductive means for closing an electric cIrcuit in one of the two stable positions of the first element relative to the second element and for opening the circuit in the other stable position. 